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Alex Sen Gupta

Observed and projected changes to the ocean, Part 1 Climate models, pitfalls and historical observations ( Chapter 3, Ganachaud et al., 2012). Alex Sen Gupta. Why do we care about the Ocean? Historical observations

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Alex Sen Gupta

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  1. Observed and projected changes to the ocean, Part 1 Climate models, pitfalls and historical observations (Chapter 3, Ganachaud et al., 2012) Alex Sen Gupta

  2. Why do we care about the Ocean? • Historical observations • Ocean temperature, stratification, sea-level, dissolved oxygen, acidification • What is a climate model? • Pitfalls • Resolution & model bias

  3. Why do we care about the Ocean? 2,500,000,000 Hiroshimas !

  4. Why do we care about the Ocean? Atmospheric CO2 Concentration What we expect What we measure • 1/4 of human CO2 emissions absorbed by ocean

  5. Historical Observations: Temperature Ocean temperature trend (1950-2010) oC/decade • Widespread warming

  6. Historical Observations: Temperature Ocean temperature trend (1950-2010) oC/decade • Widespread warming • Natural variability can mask Global Warming Ocean temperature trend (1980-2010) oC/decade

  7. Historical Observations: Temperature • Reduced salinity over last 50yr • Salinity change evidence of increased rainfall • Warming (and freshening) cause increased stratification • Implications for nutrients and oxygen Ocean salinity trend (1955-2004) oC/50yr Cravatte et al. 2009

  8. Historical Observations: Oxygen • Low oxygen zones expanding • Possibly related to reduced increased stratification Dissolved oxygen concentrations (eastern equatorial Pacific)

  9. Historical Observations: Sea-level • Global average sea-level increase ~20cm • Very rapid sea-level rise in Western Pacific over last 20 yrs • Related to natural variability PDO), not reflective of long term trend Sea-level change Combined TOPEX/Poseidon, Jason-1 and Jason-2/OSTM sea level fields

  10. Historical Observations: Acidification Ocean CO2 Build-up Carbonate/Aragonite 30% pH CO32- H+ 0.1

  11. What is a climate model?

  12. What is a climate model? Time: 1

  13. What is a climate model? Time: 2

  14. What is a climate model? Time: 3

  15. What is a climate model? Time: 4

  16. What is a climate model? Time: 5

  17. What is a climate model? Time: 6

  18. What is a climate model? Time: 7

  19. What is a climate model? Time: 8

  20. What is a climate model? Time: 10

  21. What is a climate model? Time: 11

  22. What is a climate model? Air Temperature Ocean Temperature Wind Speed Current Speed Cloudiness Water Vapour Rainfall Salinity Density Land Runoff Land Cover Ice Cover Time: 11

  23. Resolution

  24. Resolution How an Ocean model sees the ocean …

  25. Resolution How an Ocean model sees the ocean …

  26. Resolution How an Ocean model sees the ocean …

  27. Resolution How an Ocean model sees the ocean … • Broad features are captured But … • Cannot see small islands • Cannot see fine scale circulation

  28. Resolution Grid box size in the different models range from about 1° to 5°

  29. Resolution Climate Model Satellite Observations Surface Temperature Surface Temperature Gilbert Islands Gilbert Islands • Climate models can’t see small islands • So they don’t reproduce island process like upwelling

  30. Resolution Climate Model Satellite Observations Surface Temperature Surface Temperature Gilbert Islands • Models suggest that the equatorial undercurrent will strengthen • Presence of Gilbert islands reduce warming by 0.7oC

  31. Model Bias Sea surface temperature Average of all models Observations • Cold tongue extends too far to west • Warm pool isn’t warm enough • Upwelling off south America too weak

  32. Conclusions • Significant change has already occurred • But, need to be careful to separate climate change and natural variability

  33. Conclusions • Climate models successfully simulate many characteristics of the climate system • But they have their limitations

  34. Model Bias Sea surface temperature Average of all models Observations • If cold tongue is in wrong location warming might also be in wrong location Projected warming

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